A Tailored Strategy to Crosslink the Aspartate Transcarbamoylase Domain of the Multienzymatic Protein CAD

13 páginas, 5 figuras, 1 tabla
CAD is a 1.5 MDa hexameric protein with four enzymatic domains responsible for initiating de novo biosynthesis of pyrimidines nucleotides: glutaminase, carbamoyl phosphate synthetase, aspartate transcarbamoylase (ATC), and dihydroorotase. Despite its central metabolic role and implication in cancer and other diseases, our understanding of CAD is poor, and structural characterization has been frustrated by its large size and sensitivity to proteolytic cleavage. Recently, we succeeded in isolating intact CAD-like particles from the fungus Chaetomium thermophilum with high yield and purity, but their study by cryo-electron microscopy is hampered by the dissociation of the complex during sample grid preparation. Here we devised a specific crosslinking strategy to enhance the stability of this mega-enzyme. Based on the structure of the isolated C. thermophilum ATC domain, we inserted by site-directed mutagenesis two cysteines at specific locations that favored the formation of disulfide bridges and covalent oligomers. We further proved that this covalent linkage increases the stability of the ATC domain without damaging the structure or enzymatic activity. Thus, we propose that this cysteine crosslinking is a suitable strategy to strengthen the contacts between subunits in the CAD particle and facilitate its structural characterization.
This work was supported by grant RTI2018-098084-B-I00 financed by MCIN/AEI/10.13039/ 501100011033/ and “FEDER Una manera de hacer Europa”, by grant PID2021-128468NB-I00 financed by MCIN/AEI /10.13039/501100011033, and by Fundación Ramón Areces, XX National Call. F.d.C.-O. is a postdoctoral fellow of the Generalitat Valenciana (APOSTD 2021). AR-d-C is supported by salary from the European Commission–Next Generation EU through the CSIC Global Health Platform (PTI+ Salud Global) established by EU Council Regulation 2020/2094. X-ray diffraction experiments at ALBA were performed as part of the BAG proposal 2018082950.